In the area of electrothermal atomic absorption spectroscopy, electrically heated graphite furnaces are provided with tube-shaped longitudinally-heated furnace components. It is already known to provide so-called platforms in the interior of these tubes in order to delay the atomization of the specimen with respect to the electrothermal heating of the wall and of the interior of the furnace tube filled with protective gas.
These platforms are rectangular when viewed in plan and have a recess on their upper side for receiving a specimen. The platforms can be freely movable or can be fixed by direct body contact to the interior of the furnace tube or by struts formed thereon. In this connection, reference may be made to German patent publications 2,924,123; 3,545,635; 3,823,346; 3,734,001; and, DD 252,249.
The principle of the longitudinally-heated furnace tube has associated therewith the disadvantage of a temperature drop of several hundred degrees from the center of the tube to both tube ends. For this reason, the actually desired delay effect is overall reduced and limited only to a center region of constant tube temperature.
The platforms in the longitudinally-heated furnace tubes cannot be inserted and removed after each measuring operation for newly charging the same and for weighing the specimen without disturbing the entire system (the system comprising the tube and the current supply electrodes) in their existing electrical contact state. This, however, is necessary in the analysis of solid specimens.
European patent publication 0,381,948 discloses a transversely-heated furnace system wherein a platform of cylindrical shape is connected to the tube body via a strut in the vicinity of the center of the platform in order to generate the desired temperature-delay effect. Heat conduction from the wall of the tube to the platform takes place at the location of the strut which reduces the desired temperature-delay effect; that is, the temperature on the platform increases approximately as fast as the temperature of the tube wall and therefore more rapidly than the gas-phase temperature. Accordingly, the atomization of the specimen already begins before a thermal equilibrium is reached in the interior space of the tube.
A significant disadvantage of this configuration is furthermore that the spacing between the platform and the tube wall can change as a consequence of the necessary pressing pressure for the electrical contact.
This pressure acts on the surface of the tube body and because of this pressure, the tube body can be irreversibly deformed at the high temperatures which are applied which, in turn, leads to a direct contact to the platform.
In this case, the desired effect of the furnace system is cancelled.
U.S. Pat. No. 4,407,582 discloses the so-called tube-in-tube technology wherein the platform for the specimen has no direct thermal contact to the inner wall of the tube-shaped furnace body connected to the electrodes.
A conventional graphite tube is surrounded by an additional unstable heating jacket.
The impingement of radiation on the specimen is prevented by the interior tube.
The large mass to be heated and the necessary reduction of the overall diameter of the inner tube cause heat-up rates to be too long and introduce a limitation as to the area of application.
Possible unwanted transverse currents are facilitated.
The tube-in-tube technology is not utilized in commercial AAS-apparatus because of their disadvantages. Such a system is also disclosed in German patent publication 3,307,251.
German utility model registration 8,714,670 discloses a transversely-heated cuvette for use in atomic absorption spectroscopy. The cuvette has contact pieces formed laterally on the atomizer tube. A separate device for holding the specimen is not provided.